Module 2 - Cellular Basis of Inheritance

Cellular Basis of Inheritance

Study Aims and Objectives

  • To observe the morphology of chromosomes

  • To understand the processes of mitosis and meiosis

  • To analyze the relationship between meiosis and Mendel’s rule

Chromosomes

  • Definition: A thread-like structure composed of nucleic acid carrying genetic information.

  • Components: DNA, RNA, and associated proteins.

  • Structure: Usually single-stranded, doubles during mitotic division.

  • Eukaryotic Cells:

    • Composed of somatic (body) cells with homologous chromosomes inherited from parents (maternal and paternal).

    • Somatic cells are Diploid (2n), while sex cells (gametes) are Haploid (n).

    • Example in Humans: Somatic cells contain 46 chromosomes, organized into 23 pairs of homologous chromosomes, where each pair consists of one chromosome from each parent.

    • Sex Determination: The 23rd pair of chromosomes determines sex (XX for female, XY for male).

Chromosome Morphology

  • Chromatin: Complex of nucleic acid and proteins (nucleoprotein) that organizes DNA into structures.

  • Nucleosomes: Fundamental units of chromatin, involved in DNA packing.

  • Characterization Criteria:

    1. Length: Relative lengths of chromosomes.

    2. Position of the Centromere:

      • Creates two arms of chromosomes: the shorter arm called p and the longer arm called q.

    • The centromere divides the chromosome into these arms.

    • Homologous Pairs: Consists of sister chromatids (identical) and non-sister chromatids (different).

The Cell Cycle and Mitosis

  • Definition of Mitosis: A cell division process resulting in two identical daughter cells with the same chromosome number as the parent cell.

  • Phases of the Cell Cycle:

    • Interphase: G1, S, G2 phases where the cell grows and DNA is replicated.

    • M Phase: Mitosis occurs in four stages:

      1. Prophase: Chromosomes condense, centrioles move to poles, nuclear envelope disappears.

      2. Metaphase: Chromosomes align at the metaphase plate; spindle fibers connect to centromeres.

      3. Anaphase: Sister chromatids are pulled apart to opposite poles, centromeres split.

      4. Telophase: Nuclear membranes reform, chromosomes decondense, preparing for cytokinesis.

  • Cytokinesis: Division of the cytoplasm, resulting in two distinct daughter cells.

Meiosis

  • Purpose of Meiosis: Reduction of diploid (2n) chromosomes to haploid (n) for gamete formation.

  • Phases of Meiosis:

    1. Meiosis I (Reduction Division)

      • Homologous chromosomes pair (synapsis) and may undergo crossing over, leading to genetic diversity.

      • Cell divides into two haploid cells.

    2. Meiosis II (Equational Division)

      • Similar to mitosis, where sister chromatids separate, resulting in four haploid progeny cells.

  • Significance: Contributes to genetic variation through recombination and independent assortment during gamete formation.

Key Concepts in Meiosis

  • Synapsis: Homologous chromosomes pair up during Metaphase I.

  • Crossing Over: Exchange of genetic material between non-sister chromatids, enhancing genetic diversity.

  • Nondisjunction: Failure of chromosomes to separate properly during Anaphase, which can lead to genetic disorders.

Summary of the Differences Between Mitosis and Meiosis

  • Mitosis results in two identical diploid cells; Meiosis results in four genetically diverse haploid cells.

  • Mitosis involves one division; Meiosis includes two consecutive divisions (Meiosis I and II).

  • Crossing over occurs only in Meiosis, promoting genetic diversity.

Conclusion

  • Understanding cellular processes such as mitosis and meiosis is fundamental to genetics, heredity, and biological diversity. The organization, structure, and function of chromosomes play a crucial role in these processes, along with mechanisms of genetic variation through meiosis.